2202 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 42, NO. 9, SEPTEMBER 2014

Ion Beam Features Produced by Two Plasma FocusMachines Operated With Different Gases

Mohamad Akel, Sami Alsheikh Salo, Sor Heoh Saw, and Sing Lee

Abstract— Ion beams emitted from the low-inductance kJ NX2plasma focus have been numerically characterized and trends ofthese beams in various gases have been reported. In this paper,the ion beams from high-inductance kJ plasma focus machinesAECS PF2 and INTI operated with various gases are studied.The obtained results show that the beam ions mean energydepends on the gas type and increases with increasing ion massof the gas. It also indicates a higher ion fluence for H2 (4.4 ×1020 ions m−2), and a very small fluence value for Xe (0.085 ×1020 ions m−2). The ion number reduces from H2 (24 × 1014)to Xe (0.074 × 1014). The ion current decreases from H2 (20.5%of the discharge current) to 5.4% for Xe. The beam energydrops from H2 (0.63% of stored energy E0) to 0.4% for O2;however, then increases from Ne to the radiatively-collapsedgases Ar, Kr, and Xe. Argon has the highest damage factor(77 × 1010 Wm−2s0.5), while the lightest gases have the lowest(2–6 × 1010 Wm−2s0.5). The magnetic field compressing thepinch is higher for the heavier gases. This paper confirms thatthe trends of ion beam production with various gases in high-inductance machines are the same as the trends in low-inductancemachines.

Index Terms— Different gases, ion beam, Lee model, plasmafocus.

I. INTRODUCTION

PLASMA focus devices are intense sources of X-rays, neu-trons, ions, and electrons. Of interest is the use of the ion

beam for ion implantation, surface treatment, and ion-assistedcoatings [1]. Most experiments for ion beam diagnosticswere conducted not with heavy gases, but with deuterium toinvestigate the neutron production and deuteron acceleration,because the observation of the anisotropy of neutron emissionsmeans nonthermal neutron production mechanism, such as abeam target interaction, somewhat different from the resultswith light gases (hydrogen or deuterium). The pinched plasmashape related to the acceleration of deuterons. However, whena high atomic number gas, such as argon, krypton, or xenon, isused as filling gas, the plasma focus phenomena are changesfrom a long column to a short hot spot [2]. The radiationfrom the dense hot plasma may affect the plasma dynamics

Manuscript received April 7, 2014; revised June 19, 2014; acceptedJuly 21, 2014. Date of publication August 12, 2014; date of currentversion September 9, 2014. The work of S. H. Saw and S. Lee wassupported by the Research under Grant INT-CPR-01-02-2012 and GrantFRGS/2/2013/SG02/INTI/01/1.

M. Akel and S. A. Salo are with the Department of Physics, AtomicEnergy Commission, Damascus 6091, Syria (e-mail: pscientific@aec.org.sy;makel@aec.org.sy; ssalo@aec.org.sy).

S. H. Saw and S. Lee are with the INTI International University, Nilai71800, Malaysia, and also with the Institute for Plasma Focus Studies,Chadstone, VIC 3148, Australia (e-mail: sorheoh.saw@newinti.edu.my;leesing@optusnet.com.au).

Digital Object Identifier 10.1109/TPS.2014.2342743

causing radiative cooling and collapse for high Z gases, such asAr, Kr, and Xe [3]–[5]. Experimental observations suggestsAr (Z = 18) as the transition gas below which (Z < 18)the pinching and any radiative cooling effects proceed asa column. For heavier gases (Z > 18) the pinch is brokenup into a line of radiatively collapsed dense hot spots [6].The correlation between the plasma focus parameters andproduced ion beam properties could be of help to understandthe plasma surface interactions and to find the optimumconditions for desired material science applications. The Leemodel code [7] had been extended, based on the virtual plasmadiode mechanism proposed in [8] and [9], for studying of ionbeams from plasma focus [6], [10], [11]. Detailed numericalcalculations had been carried out on the ion beams from NX2operated in various gases [6]. The NX2 is a low inductancemachine specially designed for high current (200 kA/kJ) andhigh performance. Many machines available to small researchgroups are single-capacitor high inductance machines withrelatively low current (<100 kA/kJ). From the numerical workcarried out in [6] it may be surmised that the NX2, becauseof its higher current/kJ will produce considerably more ionflux and fluence than similar-energy but higher—inductancemachines, such as the INTI PF or the AECS PF2. However,it is important to show that the trends of ion beam productionin various gases are (or are not) the same for these higherinductance machines.

For this purpose, we apply the code version RADPF5.15FIBon two plasma focus devices (AECS PF2 [12] and INTIPF [7]) to characterize the ion beams emitted from these high-inductance plasma focus for various gases.

II. PLASMA FOCUS ION BEAM PROPERTIES

Lee and Saw [6] derived the flux equation of the ion beam;and computed the ion beam properties using the radiativeLee model code for the plasma focus. Since the beam exitsthe focus pinch with little divergence, the exit beam is bestcharacterized by the ion fluence defined as the number per unitcross section. Following [6], we use the following equation:

Flux (ions m−2 s−1) = 2.75 · 1015 · fe · ln(b/rp) · I 2pinch

M1/2 · Z1/2eff · r2

p · U1/2.

The ion fluence (ions m−2) is then computed by multiplyingthe flux by pinch duration τ . Here, M is the mass numberof ion, b is the cathode radius, and fe = 0.14 (the frac-tion of energy converted from pinch inductive energy intobeam kinetic energy) is equivalent to ion beam energy of3%–6% E0. The diode voltage U is U = 3 Vmax taken

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AKEL et al.: ION BEAM FEATURES PRODUCED BY TWO PLASMA FOCUS MACHINES 2203

TABLE I

FIRST COLUMN DISPLAYED THE PLASMA FOCUS MACHINES UNDER CONSIDERATION. REMAINING COLUMNS ARE THE ENERGY OF THE BANK E0,

THE ANODE AND CATHODE RADIUS a AND b, THE ANODE LENGTH z0, STATIC INDUCTANCE L0, CAPACITANCE C0 , SHORT-CIRCUITED

RESISTANCE r0 , CHARGING VOLTAGE V0, MAXIMUM CURRENT Imax , AND CURRENT RISE TIME tcr

Fig. 1. Schematic view of the plasma focus machines.

Fig. 2. AECS PF2-radius ratio versus pressure for different gases.

from data fitting [6], [10], [11], where Vmax is the maximum-induced voltage in the axial phase. The code computes the val-ues of effective charge Zeff , pinch radius rp , pinch duration τ ,pinch current Ipinch, and U from which the ion flux is deduced,followed by: fluence (ions m−2), current density (Am−2),current (A), ions per seconds (ions s−1), number of ions inbeam (ions), power density flow (energy flux) (Wm−2), energyin beam (J), and damage factor (Wm−2s0.5). The fast plasmastream FPS energy (J), plasma diode impedance (�), and theconstricting pinch magnetic field (kG) are also estimated.

III. RESULTS AND DISCUSSION

Two kJ plasma focus devices (AECS PF2 [12] andINTI PF [7]) (Table I) with the basic plasma focus parame-ters shown in Fig. 1 are used for these numerical studies.The number and energy flux and fluence in many gasesare derived to give reference values for small plasma focus

Fig. 3. (a) AECS PF2-flux versus pressure for various gases. (b) AECSPF2-flux (expanded scale) to show the heavier gases.

devices. In each gas, a range of pressures is chosen centered atthe optimum (matched) pressure at which occurs the strongestenergy transfer into the focus pinch.

The model parameters fm , fmr, and fc, fcr are mass andcurrent factors of axial and radial phases, obtained fromfitting the computed to the measured current traces. To startthe calculations, the modified Lee model code is configuredto operate as the AECS PF2 and INTI PF devices startingwith the above bank and tube parameters. The dynamics iscomputed and displayed by the Lee model code, which thencalculates the ion beam properties (ion beam energy, ion beam

2204 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 42, NO. 9, SEPTEMBER 2014

TABLE II

ION BEAM PROPERTIES GENERATED BY AECS PF2 DEVICE FOR VARIOUS GASES

flux, ion beam fluence, beam ion number, ion beam current,power flow density, and damage factor) for various gases.

A. Ion Beam Properties of AECS PF2 OperatedWith Various Gases

Fig. 2 shows the different compression of the PF pinchfor different gases. In H2, D2, and He at low pressures theradius ratio is about 0.14, the three graphs staying togetherup to 5 torr. Above 5 torr (not shown in the figure), theradius ratio of D2 and He separate from that of H2 beingslightly higher; all three graphs rising steadily to 0.17 at thehighest operational focus pressures. For N2, as observed in theNX2 [6] thermodynamics effects kick in and the radius ratiodrops from 0.12 to a value about 0.084. Similar results areobserved for O2 and Ne, these gases begin to show the effectsof radiative cooling reducing the radius ratios. Argon, krypton,and xenon show strong radiative collapse at around 1, 0.3–0.7,and 0.2–0.6 torr, respectively, all with strong focus operation.

Fig. 3(a) shows the flux in ions m−2s−1 for the variousgases. The H2, D2, and He curves showing the same trendswith H2 having 1.1 × 1028 ions m−2s−1 at 1 torr, rising

gradually to 2.3×1028 ions m−2s−1 at 10 torr before droppingas operational pressure is further increased.

The lower pressure region of Fig. 3(a) is expanded to showthe behavior of the heavier gases [Fig. 3(b)]. Nitrogen andoxygen show the same trend, but with much reduced pressurerange, peaking at 3.9 × 1027 ions m−2s−1 at 0.7 torr forN2 and 3.7×1027 ions m−2s−1 at 0.5 torr for O2. Neon showsan accentuated peak of 4.9 × 1027 ions m−2s−1 appearingat 1.4 torr corresponding to the observed radiatively collapsedcompression at 1.4 torr. Argon, krypton, and xenon showthe effect of radiative collapse peaking at highly accentuated8×1027 ions m−2s−1 at 1 torr for Ar, 4.3×1027 ions m−2s−1

at 0.3 torr for Kr, and 2.1 × 1027 ions m−2s−1 at 0.2 torrfor Xe. The accentuating effect on the flux (due to smallerradius ratio) is compensated by much greater energy perion, resulting in reduced ion numbers. Thus, we observe thatthe beam ion flux drops as the mass number of the ionsincreases, with accentuating factors provided by radiation-enhanced compression.

The shape of the curves and the trend for ion beam fluencewith gases are very similar to the flux. A detailed comparison

AKEL et al.: ION BEAM FEATURES PRODUCED BY TWO PLASMA FOCUS MACHINES 2205

TABLE III

ION BEAM PROPERTIES GENERATED BY INTI PF DEVICE FOR VARIOUS GASES

TABLE IV

SUMMARY OF RANGE OF ION BEAM PROPERTIES AND SUGGESTED SCALING

of the flux and fluence versus pressure of these AECS PF2results showing the trend with various gases and those forNX2 [6] show very similar trends except that the number andenergy fluence and flux of NX2 are generally larger due to its

larger discharge current. We proceed to a brief summary ofthe main ion production characteristics of the AECS PF2. Thebeam ion number per kJ range from 1015 for the lightest gasesto 1012 for Xe in the radiative collapse regime. Argon reaches a

2206 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 42, NO. 9, SEPTEMBER 2014

peak number per kJ of 2.6×1013. The ion currents ranges from21.3 kA for H2 at 15 torr (20% of the circuit Ipeak) to 1 kA(1% of Ipeak) for strongly compressed Xe at 0.3 torr. Thus, theion current drops with heavier gases and that radiative-collapsefurther reduces the ion current. The results further show thatalthough the beam ion number and ion current are the lowestfor the heaviest gases Ar, Kr, and Xe, yet these beams alsocarry similar amounts of energy at 0.2%–0.7% E0 comparedwith 0.2%–0.3% for the other gases due to the greater energyper ion.

However, due to the shorter pulse duration of the heavier gaspinches, the three heaviest gases produce 0.7–1.7 × 109 W ofpower flow whereas the lightest gases only carry 4–8×108 W.The damage factor reaches almost 4.2 × 1011 Wm−2s0.5 forXe, but is only 1.2 × 1010 Wm−2s0.5 for H2, with Ar havingintermediate value of 11 × 1010 Wm−2s0.5. The large valuesfor Ar, Kr, and Xe are due to the very small radius ratios of thepinch columns due to radiative collapse. The detailed resultsare tabulated for a comparative study, as shown in Table II.

B. Ion BeamCharacteristics of INTI PF for Different Gases

In the same way as described for the AECS PF2, numericalstudies are carried out on the INTI PF plasma focus devicefor various gases and the obtained results are tabulated for acomparative study, as shown in Table III.

From Table III, we note that the ratio Ipeak/a has a range of197–202 kA/cm with no perceptible trend with atomic numberof gases. The pinch length is constant (∼1.4 cm) for Ne andlightest gases, then increases for Ar and decreasing throughthe heavier gases until a value of 1.24 cm, while the pinchradius generally decreases from 0.13 cm for H2 to 0.05 for Xe.The ratio τ /a has a range of 4.1 ns/cm for Xe to 16 ns/cmfor He. The plasma diode voltage varies from 34 kV (for He)to highest value of 322 kV (for Xe) and the impedance rangesfrom 0.31 (for He) to 3.7� (for Xe). The beam ion meanenergy ranges from 44 keV (for D2) to 10678 keV (for Xe).The very high ion mean energy (computed by U× Zeff) valueof Xe is due to the large diode plasma voltage and higheffective charge state Zeff . The results indicate that the ionfluence range from 4.4 × 1020 ions m−2 for the lightest gasH2 decreasing through the heavier gases until a value of0.7 × 1020 ions m−2 for O2. For Ne the fluence increase to1.3×1020 ions m−2 due to smaller radiatively-collapsed radius.For Ar, Kr, and Xe radiative collapse is even more severe, butthere is a decrease in fluence down to 0.085 × 1020 ions m−2

due to large energy of the highly charged ion accelerated bylarge electric fields induced in the radiative collapse.

The results of the above studies of the AECS PF2 and INTIPF are summarized in Table IV, which gives the range of theion beam properties and the scaling suggested by inspectionof Tables II and III.

With this method, ion beam and fast plasma stream para-meters may be computed for any Mather-type plasma focusoperating with different gases.

IV. CONCLUSION

Numerical studies have been carried out on two low-energyplasma focus devices AECS PF2 and INTI for studying ion

beams emitted with various gases. The results showed thatthe plasma diode voltage and the impedance are higher forheavier gases. The beam ions mean energy depends on thegas type and increases with increasing of the ion mass ofthe gas. The results indicate that the ion fluence is higher forthe lightest gas, while the very small ion fluence of Xe isdue to the large energy of the Xe ion. This complex behaviorreflects small thermodynamic effects and large effects due toradiative cooling and collapse. The ion number reduces fromH2 to Xe as does the ion current from 21% to 5% of thedischarge current. The beam energy reduces from 0.63% ofE0 (H2) to 0.4% (O2) and increases slightly for the radiativecollapse gases Ne, Ar, Kr, and Xe. The damage factoris highest for Ar at 77 × 1010 Wm−2s0.5 lowest (2–6 ×1010 Wm−2s0.5) for the lighter gases.

These results using AECS PF2 and INTI PF can provide abasis for estimating effects of ion beams on target materialsespecially for the single capacitor kJ plasma focus devices,which dominate the research of small plasma focus groups.

ACKNOWLEDGMENT

The authors would like to thank the Director General ofAtomic Energy Commission of Syria for encouragement andpermanent support. They would also like to thank S. Ismael,who collaborated going through all the numerical computa-tions using Lee model.

REFERENCES

[1] H. Kelly, A. Lepone, A. Marquez, M. J. Sadowski, J. Baranowski, andE. Skladnik-Sadowska, “Analysis of the nitrogen ion beam generated ina low-energy plasma focus device by a Faraday cup operating in thesecondary electron emission mode,” IEEE Trans. Plasma Sci., vol. 26,no. 1, pp. 113–117, Feb. 1998.

[2] R. Lebert, A. Engel, and W. Neff, “Investigations on the transitionbetween column and micropinch mode of plasma focus operation,”J. Appl. Phys., vol. 78, no. 11, pp. 6414–6420, Dec. 1995.

[3] S. Lee, S. H. Saw, and J. Ali, “Numerical experiments on radiativecooling and collapse in plasma focus operated in krypton,” J. FusionEnergy, vol. 32, no. 1, pp. 42–49, 2013.

[4] M. Akel and S. Lee, “Radiative collapse in plasma focus operated withheavy noble gases,” J. Fusion Energy, vol. 32, no. 1, pp. 111–116, 2013.

[5] M. Akel, S. Lee, and S. H. Saw, “Numerical experiments in plasma focusoperated in various gases,” IEEE Trans. Plasma Sci., vol. 40, no. 12,pp. 3290–3297, Dec. 2012.

[6] S. Lee and S. H. Saw, “Plasma focus ion beam fluence and flux—Forvarious gases,” Phys. Plasmas, vol. 20, no. 6, p. 062702, 2013.

[7] S. Lee. (2014). Radiative Dense Plasma Focus Computation Package:RADPF. [Online]. Available: http://www.plasmafocus.net

[8] V. A. Gribkov et al., “Plasma dynamics in the PF-1000 device underfull-scale energy storage: II. Fast electron and ion characteristics ver-sus neutron emission parameters and gun optimization perspectives,”J. Phys. D, Appl. Phys., vol. 40, no. 12, pp. 3592–3607, 2007.

[9] V. N. Pimenov et al., “Damage and modification of materials producedby pulsed ion and plasma streams in dense plasma focus device,”Nukleonika, vol. 53, no. 3, pp. 111–121, 2008.

[10] S. Lee and S. H. Saw, “Plasma focus ion beam fluence and flux—Scalingwith stored energy,” Phys. Plasmas, vol. 19, no. 11, p. 112703, 2012.

[11] M. Akel, S. A. Salo, S. H. Saw, and S. Lee, “Properties of ion beamsgenerated by nitrogen plasma focus,” J. Fusion Energy, vol. 33, no. 2,pp. 189–197, 2014.

[12] S. Al-Hawat, M. Akel, S. H. Saw, and S. Lee, “Model parameters versusgas pressure in two different plasma focus devices operated in argon andneon,” J. Fusion Energy, vol. 31, no. 1, pp. 13–20, Feb. 2012.

[13] S. Lee, “Radius ratios of argon pinches,” Austral. J. Phys., vol. 36, no. 6,pp. 891–895, 1983.